An existing optical waveguide device will be described by exemplifying a Modulator with Polarization Beam Combiner shown in FIG. 4. For example, modulated beams of 20 Gb/s are output from output waveguides 108 and 109 of a modulator body 11. The output beam from the output waveguide 108 is input to a element to combine polarization beam (a Savart plate) 40 through a lens 50 and a prism 60. When the output beam from the output waveguide 109 passes through a ½-wave plate 30c, the polarization plane thereof rotates by 90°, and the beam is input to the element to combine polarization beam 40 through the lens 50 and the prism 60. Two beams of which the polarization planes are inclined with respect to each other by 90° are output to the same optical path by the element to combine polarization beam 40. In this way, a modulated beam of 40 Gb/s is obtained by combining the modulated beams of 20 Gb/s of which the polarization planes are inclined with respect to each other by 90°.